Grewe, T. (Corresponding author)

1999
Forschungszentrum, Zentralbibliothek Jülich

Report No.: Juel-3664

Abstract: In the development of a reactor based on controlled nuclear fusion, the tokamak is currently the most advanced concept. Tho twisting of the magnetic field lines ensures that in normal operation the hot plasma from the inner zone is confined on magnetic flux surfaces and only diffuses very slowly to the walls. A very disturbing exception to this normal case is disruptions where the magnetic confinement of the hot plasma suddenly collapses. The collapse of the plasma flux induces fluxes in the mechanical components and the associated forces may lead to destruction at the tokamak. Impingement of the particles previously enclosed by the magnetic field on the wall rnaterials leads, moreover, to high heat fluxes so that particularly exposed wall materials (limiter, divertor target plates) may melt or partially vaporize. Within the framework of this PhD thesis, the disruptive heat fluxes at the TEXTOR tokamak were investigated. The surface temperature on the ALT-Il limiter was measured by an infrared camera. A major technical innovation in this work was an improved time correlation of the IR video image with the other TEXTOR data. The absolute time correlation is a few milliseconds and the relative correlation (i.e. for short periods of time) about 0.1 ms. The local heat flux was determined from the temperature rise after disruptive heat pulses. The heat pulses last less than 100 ms; during this period the local power flux increases to more than 1000-fold that of the mean flux in the non disruptive phase. The distribution of the heat flux deposition is probably not homogeneous but it is consistent to assume a peaking factor of about 3. At the time of the disruptive heat pulses the EeE diagnosis indicates a rapid drop in temperature inside the plasma and in the plasma edge region the pulses can be observed as a transient rise with a subsequent similarly rapid drop. The impingement of the heat pulses also causes hydrogen/deuterium and impurities (e.g. carbon and oxygen) to flow into the plasma. After the heat pulse accompanied by the characteristic negative spike in the loop voltage during the thermal collapse of the plasma, further heat pulses can also be seen in the phase of flux collapse. Disruptive heat pulses represent a great danger for future fusion reactors. A further improvement in the time and temperature resolution of the infrared camera. system will provide more precise data on the duration and time course of the individual heat pulses. Together with spectroscopic studies of the cloud formed in front of the wall components during impingement of the heat pulses this could enable the measurements to be-compared with model calculations on the role of the neutral gas shielding model

Keyword(s): plasma wall interaction

1. Plasmaphysik (IEF-4)